Traditionally, successful vaccination has been dependent upon the use of live attenuated viruses or preparations of killed pathogenic organisms. These vaccines are very effective in controlling or, as in the case of smallpox, even eliminating certain infectious diseases. However, their use often present safety concerns. Subunit vaccines based on peptide or proteins derived from a pathogen are less hazardous than traditional vaccines but have generally suffered from poor immunogenicity and high expense. Moreover, current vaccines with a few exceptions must be administered parenterally. However, it is well known that most pathogens gain entry across the mucosal surfaces of the body and a mucosal immune response would therefore be more appropriate.
Both safety concerns and the desire to target mucosal tissues for more effective immunization against common pathogens dictate the need for new approaches to vaccination. For induction of a mucosal response, oral administration of antigen is appropriate, inexpensive, and safe. However, in order to efficiently immunize by the oral route, several obstacles such as degradation from low pH or proteases in the gastrointestinal GI tract, the short exposure to immune induction sites, and limited permeability must be overcome.
Recent studies demonstrate that plants and plant viruses can function as effective tools for vaccine production and delivery. Furthermore, like liposomes and microcapsules, it is expected that plant cells and plant viruses will serve as delivery vehicles providing natural protection for the antigen associated with them and enhancing the uptake of the antigen from the GI tract. Such new developing "green system vaccines" have significant advantages over the traditional and synthetic vaccines as regards safety, deliverability via either parenteral, nasal or oral routes, and lower cost of production.